This invention relates to a fuel injection control apparatus for an internal combustion engine, and in particular to a fuel control apparatus for processing the measured values of the inlet air flow rate of an internal combustion engine for an automobile.
Heretofore, there has been proposed such a fuel control apparatus for an internal combustion engine as shown in FIG. 1. In the figure, an internal combustion engine 1 is supplied with fuel by an electromagnetically driven injector 2. A hot-wire type air flow sensor (hereinafter abbreviated as AFS) 3 for sensing the flow rate of an inlet air inhaled into the engine 1 and a throttle valve 5 for adjusting the flow rate of the inlet air into the engine 1 are mounted on the inlet pipe 6 as shown in FIG. 1. A water (coolant) temperature sensor 7 is also disposed near the engine 1 to indicate the temperature of the engine 1. An ignition control unit 8 computes a fuel amount to be supplied to the engine 1 from an air flow rate signal obtained by the AFS 3 and applies to the injector 2 pulses whose pulse widths correspond to a required fuel amount. The ignition control unit 8 is connected to a well known ignition device 9 which generates an ignition pulse signal each time the engine 1 is at a predetermined rotational angle. Also disposed in this fuel control apparatus are a fuel tank 11, a fuel pump 12 for pressurizing the fuel, and a fuel regulator 13 for maintaining a constant pressure on the fuel supplied to the injector 2, as is well known in the art.
The ignition control unit 8 includes an input interface circuit 80, a micro-processor 81 for processing various input signals from the input interface circuit 80, computing a fuel amount to be supplied to the inlet pipe 6 of the engine 1 in accordance with a program previously stored in a ROM 82, and for controlling the driving signal of the injector 2, a RAM 83 for temporarily storing data during the process of the computation of the micro-processor 81, and an output interface circuit 84 for driving the injector 2.
In the operation of the fuel injection control apparatus for an engine shown in FIG. 1, in the well known manner, the control unit 8 receives as an input an inlet air flow rate of the engine 1 detected by the AFS 3, calculates a fuel amount to be supplied to the engine 1 on the basis of the detected flow rate, detects the rotational speed of the engine 1 from the ignition pulse frequency provided by the ignition device 9, calculates a fuel amount per one engine revolution, and applies pulses with a required pulse width to the injector 2 in synchronization with the ignition pulses. It is to be noted that since the air/fuel (hereinafter abbreviated as A/F) ratio required for the engine 1 needs to be preset at the rich side when the temperature of the engine 1 is low, the pulse width of the pulses applied to the injector 2 may be incrementally corrected in accordance with thermal signals obtained from the coolant temperature sensor 7.
Since the AFS 3 used for this fuel control apparatus can detect the inlet air flow rate by the weight thereof, it has an excellent feature that there is no need to additionally provide a correction means for changes in the atmospheric pressure. However, the AFS 3 is quite sensitive to an air blow-back phenomenon caused by the overlapped operation of the inlet and exhaust valves of the engine whereby the AFS 3 detects an inlet air flow rate signal including the blow-back flow rate so that it erroneously develops an output signal indicative of a flow rate larger than the actual inlet air flow rate.
The aforementioned blow-back phenomenon may easily arise during low speeds of the engine and in a condition where the throttle valve of the engine is fully opened, where the true inlet air flow rate assumes such a waveform as if the inlet air flow rate has increased as shown in FIG. 2, despite the fact that no inlet air is inhaled during a time interval Tr.
As a result, as shown in FIG. 3, the output of the AFS 3 exhibits a value considerably higher than the true value (shown by dotted lines) during a low speed zone (or region) and in the fully opened condition of the throttle valve. Dependent on the layout of the engine or the inlet air system, an error due to the blow-back phenomenon may attain as much as a 50% increase of the true value so that such an AFS can not be made practical without any modification thereof.
In order to compensate for such an error, there has been proposed a system in which the output signal "a" shown by the arcuate portion of a solid curve in FIG. 4 provided by the AFS 3 is neglected. Instead a clipping value "c" (average value), shown by a dotted line in FIG. 4, which is somewhat larger (by e.g. 10%) than a value "b" (actual value) of the true inlet air flow rate of the engine 1 is determined by reading from the ROM 82 the maximum inlet air flow rate (including some variation) corresponding to speed of the engine 1, which was previously stored in the ROM 82.
This operation based on the concept of FIG. 4 is illustrated in the flow chart shown in FIG. 5. Namely, at first, an inlet air flow rate (Qa) is read in by the AFS 3 and an engine speed (Ne) is read in by the ignition device 9 (step T1 and T2). It is then checked in step T3 whether or not Qa&gt;c(Ne), i.e. whether or not Qa is larger than the clipping value c(Ne) which is a function of the engine speed Ne. If the answer is "yes", then the clipping operation is made in step T4 so that the inlet air flow rate is clipped to c(Ne). If the answer is "no", then no clipping operation is made as illustrated in step T5 so that the inlet air flow rate Qa is directly used. Then, the pulse width of the pulse to be applied to the injector 2 is calculated in step T6 according to the well known equation: To=KxQ/Ne where K is a predetermined constant.
However, according to this system, the clipping value "c" shown in FIG. 4 for the inlet air flow rate is preset at maximum inlet air flow rate for the engine 1 being at sea level, and therefore, an A/F ratio for a low atmospheric pressure when a car is being driven at a higher altitude should be largely shifted towards the rich side, resulting in a possibility of not only wasting fuel but also inducing a misfire.
On the other hand, another correction system of subtracting a blow-back waveform from the inlet air waveform has also been proposed. However, the blow-back waveform gradually varies relative to the opening of the throttle valve and the engine speed so that the discrimination between the blow-back waveform and the inlet air waveform can not be precisely made. One example of this system is disclosed in Japanese Patent Application Laid-open No. 56-108909 published Aug. 28, 1981. This publication describes an air flow rate detector in which a hot-wire type AFS is used to detect the inlet air flow rate by correcting an error due to the blow-back air flow rate.
In such a fuel injection control apparatus for an internal combustion engine thus arranged, a disadvantage is that the hot-wire type AFS used therein erroneously detects the inlet air flow rate to be higher than the true value due to the air blow-back phenomenon arising during low engine speed and in the fully opened condition of the throttle valve due to the overlapped operation of the valves of the engine so that an operating zone exists where the A/F ratio can not be properly controlled.